Light valves are a key element in projection displays. Preferably, such light valves are activated directly by video or data signals, using matrix addressing, or shift registers in combination with matrixing, to input the signals. Liquid crystals and electrochemichromic material have been studied as appropriate light valve media but liquid crystal (LC) has gained greater acceptance to date. For a definition of electrochromic and electrochemichromic material see, I. F. Chang, Nonemissive Electro-Optic Display, edited by A. Kmetz and F. K. Von Willisen, Plenum Press, 1978. Although a variety of drive circuit techniques are under study, active silicon integrated circuit drivers have been shown to be compatible with both liquid crystal and electrochemichromic media and thus presently represent a highly desirable approach for either a direct view or a projection display application (see articles by L. T. Lipton et al, SID Symposium Digest, 8, 64-65, 1977 and D. J. Barclay, et al, SID Symposium Digest, 11, 124-153, 1980). However, the complexity and size of the required silicon chip contribute to high cost and resolution limitations.
An alternative that has received considerable attention, (e.g. see L. T. Lipton et al, SID Symposium Digest, 9, 96-97, 1978), is a valve that uses light addressing or activation, also called a light-activated light valve (see W. P. Bleha et al, Proc. SPIE 317, 179, 1981). In this case, an optical image is applied to the photosensitive back side of the light-activated light valve (LALV) and the light intensity is used to vary the local AC voltage applied across an LC cell. The local cell reflectivity is a function of the magnitude of the cell voltage. In an early implementation, a Se or CdS photoconductive layer was used as the photosensing element that varies the AC voltage, applied to the LC, point by point. The exciting image was produced by a CRT coupled to the photoconductive layer by a lens or a fiber optic plate. A more recent implementation involves a version of the LALV in which the photoconducting layer is replaced by a layer of SiO.sub.2 on a single crystal Si as the photodetecting control element (see V. Efron et al, SID Digest of Technical Papers, 142, 1981). A microdiode grid serves to isolate the resolution elements. The resulting MOS diodes are unilateral and must be recharged. Hence, the sign of the voltage across the LC is unchanged, and the device operated in a DC mode. Earlier work used a dynamic scattering mode in the liquid crystal. More recent efforts have utilized a nematic liquid crystal operated in a voltage controlled birefringence mode.
The photoconductivity-controlled, AC liquid crystal LALV is, in principle, a good device. However, photoconductivity generally offers a poor response time-sensitivity tradeoff. Photoconductors exhibit lag which is light level dependent, a nonlinear response (gamma.perspectiveto.1/2), and are easily damaged by bright light. They exhibit burn-in effects. Photoconducting material has applications limited generally to light sensing. Consequently, a thorough study of the material system, fundamental to its performance characteristics and reliability, may be difficult to justify and is often lacking. The silicon diode version is limited to DC operation.
The present invention is directed generally to replacing the photoconductive and associated light blocking and reflecting layers of the LALV with a monolithic silicon chip containing an array of junction photodiodes and providing an AC voltage across the liquid crystal layer. Prior art examples of such an approach are found in the above-noted V. Efron et al, SID Digest of Technical Papers, 142, 1981, wherein a photoconductor is replaced with a silicon diode array for implementation in vidicons, and in I. F. Chang et al, SID Symposium Digest 102, 1973; Proc. of SID, 16,227, 1975, disclosing an electron beam addressed storage CRT. Since the silicon photodiode array can be addressed by an electron beam directly or via a phosphor coupling, the photodiode array controlled AC liquid crystal light valve can be made as a faceplate for a miniature CRT. Some of the advantages of this approach are: AC liquid crystal cells are more reliable; the interface between the liquid crystal layer and the silicon control structure can be dielectric, suitably processed for liquid crystal alignment; AC operation provides a convenient charge/discharge mechanism, allowing a transient response; and electron beam addressing is an effective way of addressing a high resolution light valve.